208 lines
7.5 KiB
Java
208 lines
7.5 KiB
Java
/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent.atomic;
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import java.lang.invoke.MethodHandles;
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import java.lang.invoke.VarHandle;
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/**
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* An {@code AtomicMarkableReference} maintains an object reference
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* along with a mark bit, that can be updated atomically.
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*
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* <p>Implementation note: This implementation maintains markable
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* references by creating internal objects representing "boxed"
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* [reference, boolean] pairs.
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*
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* @since 1.5
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* @author Doug Lea
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* @param <V> The type of object referred to by this reference
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*/
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public class AtomicMarkableReference<V> {
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private static class Pair<T> {
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final T reference;
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final boolean mark;
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private Pair(T reference, boolean mark) {
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this.reference = reference;
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this.mark = mark;
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}
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static <T> Pair<T> of(T reference, boolean mark) {
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return new Pair<T>(reference, mark);
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}
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}
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private volatile Pair<V> pair;
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/**
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* Creates a new {@code AtomicMarkableReference} with the given
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* initial values.
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*
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* @param initialRef the initial reference
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* @param initialMark the initial mark
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*/
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public AtomicMarkableReference(V initialRef, boolean initialMark) {
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pair = Pair.of(initialRef, initialMark);
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}
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/**
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* Returns the current value of the reference.
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*
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* @return the current value of the reference
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*/
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public V getReference() {
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return pair.reference;
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}
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/**
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* Returns the current value of the mark.
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*
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* @return the current value of the mark
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*/
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public boolean isMarked() {
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return pair.mark;
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}
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/**
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* Returns the current values of both the reference and the mark.
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* Typical usage is {@code boolean[1] holder; ref = v.get(holder); }.
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*
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* @param markHolder an array of size of at least one. On return,
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* {@code markHolder[0]} will hold the value of the mark.
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* @return the current value of the reference
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*/
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public V get(boolean[] markHolder) {
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Pair<V> pair = this.pair;
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markHolder[0] = pair.mark;
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return pair.reference;
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}
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/**
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* Atomically sets the value of both the reference and mark to the
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* given update values if the current reference is {@code ==} to
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* the expected reference and the current mark is equal to the
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* expected mark. This operation may fail spuriously and does not
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* provide ordering guarantees, so is only rarely an
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* appropriate alternative to {@code compareAndSet}.
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*
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* @param expectedReference the expected value of the reference
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* @param newReference the new value for the reference
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* @param expectedMark the expected value of the mark
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* @param newMark the new value for the mark
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* @return {@code true} if successful
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*/
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public boolean weakCompareAndSet(V expectedReference,
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V newReference,
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boolean expectedMark,
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boolean newMark) {
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return compareAndSet(expectedReference, newReference,
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expectedMark, newMark);
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}
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/**
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* Atomically sets the value of both the reference and mark
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* to the given update values if the
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* current reference is {@code ==} to the expected reference
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* and the current mark is equal to the expected mark.
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*
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* @param expectedReference the expected value of the reference
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* @param newReference the new value for the reference
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* @param expectedMark the expected value of the mark
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* @param newMark the new value for the mark
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* @return {@code true} if successful
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*/
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public boolean compareAndSet(V expectedReference,
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V newReference,
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boolean expectedMark,
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boolean newMark) {
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Pair<V> current = pair;
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return
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expectedReference == current.reference &&
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expectedMark == current.mark &&
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((newReference == current.reference &&
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newMark == current.mark) ||
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casPair(current, Pair.of(newReference, newMark)));
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}
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/**
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* Unconditionally sets the value of both the reference and mark.
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*
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* @param newReference the new value for the reference
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* @param newMark the new value for the mark
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*/
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public void set(V newReference, boolean newMark) {
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Pair<V> current = pair;
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if (newReference != current.reference || newMark != current.mark)
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this.pair = Pair.of(newReference, newMark);
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}
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/**
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* Atomically sets the value of the mark to the given update value
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* if the current reference is {@code ==} to the expected
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* reference. Any given invocation of this operation may fail
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* (return {@code false}) spuriously, but repeated invocation
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* when the current value holds the expected value and no other
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* thread is also attempting to set the value will eventually
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* succeed.
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*
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* @param expectedReference the expected value of the reference
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* @param newMark the new value for the mark
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* @return {@code true} if successful
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*/
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public boolean attemptMark(V expectedReference, boolean newMark) {
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Pair<V> current = pair;
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return
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expectedReference == current.reference &&
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(newMark == current.mark ||
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casPair(current, Pair.of(expectedReference, newMark)));
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}
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// VarHandle mechanics
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private static final VarHandle PAIR;
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static {
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try {
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MethodHandles.Lookup l = MethodHandles.lookup();
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PAIR = l.findVarHandle(AtomicMarkableReference.class, "pair",
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Pair.class);
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} catch (ReflectiveOperationException e) {
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throw new ExceptionInInitializerError(e);
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}
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}
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private boolean casPair(Pair<V> cmp, Pair<V> val) {
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return PAIR.compareAndSet(this, cmp, val);
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}
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}
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